I'm sitting getting ready to purchase the aforementioned DS416Play to replace a slightly failing (USB backup of the NAS has failed) and non expandable WD4TB NAS which is about 95% full but also is a single disk NAS.

After much research I settled on the above unit after comparing features of other manufactures and models, specifically in part for the SHR option (though that applies to all / most synology units) for future upgradability and as such will only put 3x4TB drives in place leaving a slot for future growth with larger drives if I ever get close to doubling my current data needs. This provides a much needed space upgrade, drive redundancy and I will also setup a backblaze online backup once I've migrated to the Synology (hopefully from the NAS->backblaze directly). I'll also be getting a UPS to prevent hard power offs to the device which should help it keep a bit more healthy for longer! I feel that's a happy setup for a home user who keeps the NAS as the main file location for movies / music and a general file dump (I do some video editing and multi track audio recordings) as I never know which device (multiple PCs / MACs / iDevices) I'll pick up to use said files. I also like the idea of link aggregation and will, at some point get my mikrotik 24port router setup to enable that. (That's my basic operating info so you all know the background to the new build).

Having settled on the standard (not pro) version of the seagate Iron Wolf drives due to the built in health management software, I was wondering how best to initialise the drives, with some online repositories suggest a burning in phase to test the drives prior to use? Is that something that would be recommend? What does that entail?

I was initially toying with the idea of getting a single 4TB iron wolf, later adding the other 2x4TB drives over a few weeks to spread the cost, but that would mean I wouldn't be able to 'burn' test the latter drives prior to use. Is that an issue? Understanding also that i won't get drive redundancy till the 2nd drive is installed, and won't achieve the 8TB SHR amount til drive 3 is installed.

Finally, if I do buy all 3 drives at once which I'm also toying with the idea of so it's a single setup rather than multiple bouts to it, they are likely to have a similar mean time till failure due to being same model, same age drives. Anything else I could do about that? I've seen it recommended to try and get different models / age of drives to prevent simultaneous / similar fail times, especially if one drive goes down the rebuild phase may put additional strain on the non-replaced drives and put them into fail territory.

I'm hoping this system will, once setup and tinkered with to get desired functionality, sit in the background and not need much of my attention till it's full (assume 5-10yrs at current data growth rate) or a drive gets unhealthy or worse still, a complete failure of the box (DS416Play failure rather than drive!) and a loss of the array. I don't mean I'll never have to touch it till the above happens, but in the mainstay be maintenance light and not require repeated tinkerings with.

To Recap

1, Do you recommend a 'burn in' test of new systems? If yes what does that entail.
2, Is buying multiple drives of same model age a really bad idea? If yes, and wanting to stick to the iron wolf drive for the enhanced software monitoring, what can I do about that?
3, Am i just being a little to nerotic? Should I just buy all three Iron Wolf drives at once and crack on without a burn?

You will receive answers than will contradict eachother, based on each ones experience.
I, for ex, bought HDDs at once and had no problems. The oldest ones are 9yo by now and still working.
I don't run stress tests on HDDs simply because I'm doing backup and I don't really care if I loose a RAID.
As so many people are saying, RAID is not backup! Data on a RAID can be lost for technical reasons (bad HDD batch, bad motherboard) or bad luck (user action, fire, water, theft) so it doesn't really matter how much stress tests you would run, it is better to safeguard your important data.

If you really want to do burn test on HDDs, there's no good way build in Synology. You need an USB caddy and run vendor tool on a PC/laptop.

That said: Get three identical drives, and buy a fourth to have on hand as a spare. If one of your three drives shows signs of trouble at some point, you will want to immediately swap in an identical drive to replace it. If you never have to use the spare, who cares; drives are cheap. Make absolutely sure that the drives you choose are listed on Synology's list of compatible drives for your DS416play.

Burning in isn't a bad idea, and I would put each drive on a SATA interface in a desktop and run Spinrite on it at Level 2; this will thoroughly check the drive for any bad sectors, and mark them, and of course give the drive a pretty solid workout.